1. Field of the Invention
The present invention relates to a power supply into which a zero-cross detection circuit usable for controlling an electronic apparatus is loaded.
2. Description of the Related Art
As an example of a fixing apparatus that is loaded into an image forming apparatus such as a copying machine or a printer, an apparatus including an endless belt (or an endless film) and a ceramic heater that contacts an inner surface of the endless belt, and a pressure roller that forms a fixing nip portion together with a ceramic heater across the endless belt. As a control method for supplying electric power to the fixing apparatus, phase control of electric power supplied from an alternating current (AC) power supply to the ceramic heater can be performed using a switch element such as a thyristor (triac). In order to perform phase control of electric power supplied from the AC power supply, timing at which an AC voltage becomes 0 V (this timing is hereinafter referred to as a zero crossing) needs to be accurately detected as the basis for the phase control.
In recent years, power consumption (also referred to as standby electricity) in a state where the image forming apparatus does not perform an image forming operation (hereinafter also referred to as a sleep state) should be further reduced. To further reduce the power consumption in the sleep state, electric power consumed by a zero-cross detection circuit needs to be reduced. This is because the supply of electric power to the zero-cross detection circuit is continued even in a state where the image forming apparatus does not operate. Therefore, to prevent the zero-cross detection circuit from consuming electric power in the sleep state, a circuit for cutting off a current for detecting the zero crossing needs to be provided.
Japanese Patent Application Laid-Open No. 2003-199343 discusses a power supply circuit including a zero-cross detection circuit and a capacitor (hereinafter referred to as a Y capacitor) serving as a capacitive component between a potential obtained by full-wave rectifying an AC voltage from an AC power supply, and a frame ground (hereinafter referred to as FG). In the power supply circuit, a resistor for discharging the Y capacitor (hereinafter referred to as a Y capacitor discharge resistor) is required to accurately detect a zero crossing. If the conventional zero-cross detection circuit is used, therefore, a current flowing through the zero-cross detection circuit and the Y capacitor discharge resistor needs to be cut off to reduce the power consumption in the sleep state of the image forming apparatus. Thus, a plurality of cutoff circuits needs to be provided. If the cutoff circuits are provided, a circuit cost and a circuit scale are increased.
On the other hand, the power supply circuit may include a capacitor serving as a capacitive component (hereinafter referred to as an X capacitor) provided between lines to which electric power is supplied from an AC power supply. The X capacitor is generally provided as a measure against noise. When a user pulls out a power supply cable that supplies electric power to a power supply circuit, the X capacitor may be electrically charged by the AC power supply. When the user pulls out the power supply cable, the user may contact a terminal of an outlet. Therefore, a discharge resistor for discharging the electric charge of the X capacitor (hereinafter referred to as an X capacitor discharge resistor) is required.
Therefore, in the power supply circuit, a state of the power consumption is required to be switchable to further reduce the consumption in a simple configuration in the sleep state and a state where the zero crossing of the AC voltage is detected.